Like several other groups, we have proposed that the CA3 region of
the hippocampus uses the NMDA channels in recurrent collaterals to
store sequences of events (places) and thereby serve as a
heteroassociative network. We have specifically argued that the
readout of these sequences at a rate of one memory per gamma cycle
can account quantitatively for the "phase-advance" of hippocampal
place cells. One problem with this proposal is that the
phase-advance is also observed in dentate granule cells. We now offer
a possible answer to this problem. Theoretical work has suggested
that the best method for predicting sequences is to have each
step-wise prediction "cleaned up" by an autoassociative network
before it serves as a cue in the next step of sequence prediction. We
propose that the dentate-hilar region is the autoassociative network
that accomplishes this task. Two anatomical findings are relevant.
First, it is known that CA3 axons have a branch that turns backward
and innervates cells in the dentate/hilar region. Second, although
dentate granule cells do not have recurrent axons as required in the
simplest autoassociative networks, they do innervate hilar mossy
cells. These in turn, make recurrent connections onto granulate
cells. This disynaptic circuit could allow the dentate/hilar region
to perform autoassociative function. The interaction of an
autoassociative network in the dentate/hilar region and a
heteroassociatve CA3 network could produce accurate sequence recall.